Owning a physical painting grants access to the object, so for example you can study the texture of the brush strokes, the canvas, take samples of the paint, smell it, destroy it, produce copies or images of it at any level of detail or resolution you like. None of this is possible with a digital reproduction.

If I own the copyright of an artwork, even one 'on the internet' I have exclusive rights to reproduce it commercially. For example I could sell prints, while it would be illegal for others to do so. The existence of the latest pop songs on Youtube doesn't stop the copyright holders commercialising it.

NFTs don't by themselves confer any of these rights or privileges with respect to the thing they purportedly represent.